As an invention related to a conventional common-mode choke coil, a common-mode noise filter described in, for example, Japanese Patent Laid-Open Publication No. 2005-56934 is known. The common-mode filter has a first wire wound around a drum core and a second wire wound over the first wire.
However, the common-mode choke coil described in Japanese Patent Laid-Open Publication No. 2005-56934 might not be able to effectively remove common-mode noise. FIG. 4 provides graphs showing the relationship between positions along the first wire and potential and the relationship between positions along the second wire and potential.
Since the common-mode choke coil has the second wire wound over the first wire, the second wire is longer than the first wire. In this case, when differential-mode signals are transmitted through the first and second wires, the potential at one end of the first wire and the potential at one end of the second wire are equal in absolute value, as shown in FIG. 4, but the potential at the other end of the first wire and the potential at the other end of the second wire are not necessarily equal in absolute value. As a result, the differential-mode signals are outputted as common-mode noise.